The essence of working principles of equipment serving for segregation of solid substances is known per se and involves heat transfer. The greatest problem being, as is well known, that the solid substance forming on the heat transmitting surface, leaves it partly only, and the remaining part adheres on the surface. Depositions in equipment working on a higher than freezing point temperature, e.g. in crystallizing equipment for solid matter, and in equipment working on a lower than the freezing point temperature, e.g. in equipment operating by freezing, ice formation, significantly hinder the heat transfer, and in extreme cases may also necessitate stopping of working.
In hindering heat transfer by depositions, it is characteristic that while the heat conduction coefficient is for copper 320, for aluminum 200, for steel 15 to 50, in case of boiler scale deposited on the walls the coefficient is 0.1 to 2.0, for ice 1.5 to 2.0 and for various gels and salts 0.3 to 0.5 kcal./m.sup.2.h..degree.C. (See: VDI Warmeatlas continuous edition).
Taking into account that the thickness of the layer covering the wall is of the same order of magnitude as the wall thickness, or even greater, the harmful effect of the layer on heat transfer, and consequently the difficulty of continuous working caused by deposits, would be obvious.
Processes known so far try to prevent deposits by increasing the circulating speed of the solution, by diminishing the temperature difference between cooled and cooling media, or by using mechanical scraping devices. For example, the known "Oslo" or "Krystal" type crystallizers work with 1 to 2 m./s. flowing speed and max. 2.degree. C. temperature difference in the cooling pipes for 120 to 150 hours without cleaning. Although the heat transfer coefficient may reach 800 kcal./m..sup.2.h..degree.C. value, because of the low temperature difference, however, rather large surfaces are needed for cooling. For example, characteristics of the equipment may be: the heat (product of the heat transfer coefficient and of the temperature difference) transferred in one hour on a surface of 1 m..sup.2 maximally can be at least 1 600 kcal.
Crystallizers with mechanical scrapers (Swanson-Water), crystallizers with double pipe, worm crystallizers, may work also for a longer time without cleaning, their heat transfer coefficient is however, at least 100 kcal./m.sup.2 h.degree.C. For these there is no significant requirement relating to the temperature difference, so e.g. with 20.degree. C. temperature difference on 1 m.sup.2 surface in an hour 2,000 kcal. can be transferred.
In these processes measures are taken for preventing deposits by high circulating relations (1:50 to 1:250), by driving the scraping mechanism, but due to the high hydraulic resistance (30 to 40 atm.) and high energy requirement, they are expensive to operate, so that no complete solution is achieved, because the equipment has to be cleaned too often.
A newer process, achieved by the crystallization equipment of J. Cerny operates with direct heat transfer, realized by a fluid not mixing with the crystallizing solution, eliminating by this the use of a rigid heat transfer surface. With its application, however, formation of inclusions and contamination of the crystallized product or of the solution, occurs, therefore it can be used but in a confined region only.